Transmitting apparatus for communication systems using frequency modulation



Patented Aug. 9, 1949 TRANSMITTING APPARATUS FOR COMMU- NICATION SYSTEMS USING FREQUENCY MODULATION Adrian R. Doucette, Swissvale, and Paul E. Lorentzen, Pittsburgh, Pa., assignors to The Union Switch and I a., a corporation Signal Company, Swissvale, of nsy ania Application April 5, 1947, Serial No. 739,628 5 Claims. (Cl. 332-19) Our invention relates to transmitting apparatus for a communication system using frequency modulation. and more particularly to transmitting apparatus for an inductive carrier communication system us ing frequency modulation.

A reactance tube modulator is frequently used in the transmitter, that is, in the transmitting apparatus of a communication system using frequency modulation. Such an arrangement generally includes inductance or choke coils in the input of the modulator and between the modu lator and the oscillator. These devices may reduce the sensitivity of modulation. That is, the amount of deviation (modulation) of the center carrier frequency for a given unit of the signal or modulating current may be reduced. A Further more, such choke coils are of relatively large and expensive construction. In portable units for communication systems of thetype here involved, it is important to reduce the number of parts required and to simplify and minimize theirconstruction and size in order that the unit can easily be handled. Also, high modulation sensitivity and a high order of frequency stability are requisites for such units.

Accordingly, a main object of our invention is the provision of improved transmitting appa atus to communi a n ys e s us n eq e i i edul ion- A more specific object of our invention is the provision of an F. M. transmitter in orporating novel means to give high modulation sensitivity tabi i and a high order of frequency Again, a feature of our invention is the 13, 70: :3.

vision of an F. transmitter incorporatin g a novel automatic frequency control and jmethqd of applying voice frequencies to the modulator.

Specificially, a feature ,of our invention is the provision of transmitting apparatus for a portable unit of an inductive carrier railway traincommunication system using frequency modulation. Other features, objects and advantages of our invention. will appear as the specificationprogrosses.

The foregoing objects, features and advantages of our invention are attained by the provision of .an oscillator-modulator in which high frequency inductance or ,chokeooils are eliminated. This gives a high sensitivity of modulationarid reduces the size andcost.of. construction.

For frequency stability an automatic frequency c ntr is d d y takin a v ia f m .ihle osci ato a d ap l insit we :bufie i e to t i mm; o wh ch line i gener ted a resistan e load from which the carrier is filtered. Voltage appl ed to the buffer tube is amplified and limited and applied to a discriminator network. The voltage output from the discriminatornetwork and a Voltage taken from the resistance load are combined and applied to the grid of the modulator to affect automatic frequency control of the oscillator. Audio voltages created in a micro? phone are applied to a p e-emphasis circuit which is coupled to the grid of a modulator tube in multiple with the output of the discriminator to forman adio frequency load for the modula tion cf the carrier frequency. This pre-emphasis circuit also serves as a filter to shunt theau'dio frequency voltages appearing at the output of a discriminator.

For a better understanding of our invention reference may be had'to the accompanying drawing, whichis a diagrammatic view showing'a preferredconstruction of our invention when used with a portable unit of an inductive carrier communi ation svst m- It is to be understood that our invention is not limited to a portable unit and this one application serves to illustrate the utility of themvention.

Referring to the drawing, the reference character ,PU' indicates a power unit made up of a low voltage battery lll and a tuned alternator TA, the alternatoribeingused to convert the low direct voltage from the battery into a unidirectional voltage for energizing the anode circuits of the electron tubes.

The battery I!) may be of any suitable voltage such as a six-volt battery and the tuned alternator TA may be of any one of several well-known types.

It is sufiicient for this application .to point out that .an operatingmagnet H of .the tuned alt'er nator is energized from battery [0 to attract a tuned. ontact'meinber'ld to ,engagea contact I? and shunt'the magnet I]. This sets the tuned contact member]! into vibration, the rate of vibration being determined by the proportioning of the member 1.5!. Vibration of member 14 to alternately engage contacts l2 and I5, causes current from' battery' IE! to alternately'flowin the two half portions of primary winding" l3'jof a transformer Tl and induce an alternating voltageinjsecondary"winding l5 of the transformer. The frequency 'of this alternating voltage is determined by the rate of vibration of the member L4. I'I'he outside terminals of secondary winding L5 arenonnectedto contacts I?! and 18 which are alternately engag'edby the member l4 'and the it alternating voltage is rectified to create a undirectional voltage between mid terminal of winding IE and ground electrode IS connected to the pivot point of the vibrating member I4, the mid terminal of winding I5 being the positive terminal.

An oscillator 080 comprising an electron tube V2 and associated circuits, is provided. The tube V2 is disclosed as a pentode but other types of tubes can be used. Filament 20 of the tube V2 forms the cathode, a mid point of the filament being connected to an auxiliary grid 23 in the usual manner. The filament 2b is heated from battery Hi, the filament 20 beingincluded in a filament circuit along with the filaments of two other tubes Vi and V3, to be referred to later, and which filament circuit can be traced from the positive terminal of battery It through ground electrodes I9 and 2 filament 26 of tube V2, wire 3|, filament 32 of tube V3 and resistor 33 in multiple, wire 3%, the two half portions of filamerit 28 of tube VI in multiple, and wire 35 to the negative terminal of battery I0. An anode-cathode circuit for tube V2 is powered from the power unit, the circuit extending from the mid terminal of winding IE of the transformer TI through a choke coil 40, a tuned primary winding 2| of an output transformer T2, anode 22 and tube space to cathode 2!], ground electrodes 24 and 19, contact member M of the tuned alternator, and alternately through the contacts I1 and IB to the outside terminals of winding it.

An oscillatory circuit comprising an inductor LI and a capacitor CI is connected across grids 25 and 26 of tube V2, a capacitor CIES being interposed in the connection to grid 26 and a resistor RI being disposed between grid 2t and round. The parts are proportioned so that oscillations of a predetermined carrier frequency having a center frequency of say, frequency II, are created. The frequency of the oscillations is primaril determined by the inductance LI and capacitance C2, but it is also controlled by the reactance of a modulator tube Vi connected to the oscillatory circuit in the usual way.

The modulator tube VI is shown as a pentode similar to the oscillator tube V2. The anode circuit of tube V! is powered from the power unit, its anode 21 being connected to the mid terminal of secondary winding I6 of transformer TI through a portion of the inductor LI of the oscillatory circuit, and its cathode 28 being connected to ground through the filament circuit of the tubes. Voltage from the power unit is applied to grid 29 of tube Vi through a resistance RIZ having a by-pass capacitor CI I. Also, a capacitor C2 is connected between the anode 21 and the control grid 36 of the tube Vi. It is clear that voltage variations applied to control grid 3!] of tube V! to cause corresponding variations in the reactance of the tube will in turn serve to frequency modulate the oscillations created by the oscillator. Furthermore, it is clear that the modulation of the carrier frequency will be repeated in the current supplied to a load circuit connected to secondary winding AI of the output transformer T2.

For automatic frequency control, a voltage is taken from the oscillator through a capacitor CIO and applied to a control grid 33 of tube V3, resistors R9 and RH) in series being connected between grid 36 and ground to form a load circuit ior the input of the tube V3. The tube V3 is disclosed as a pentode and this tube serves as a buffer tube. Anode 37 of tube V3 is powered from ami o the power unit through resistor R1 and winding L4 of a discriminator network, to be described shortly. Also, a voltage is applied to grid 38 of tube V3 from the power unit through resistors R? and R6 in series, a capacitor CI3 being connected between the junction terminal of resistors R? and R6 and ground and a capacitor CI5 being connected between grid 38 and ground. Thus the voltage app-lied to control grid 36 of tube V3 is amplified and limited in the anode circuit of this tube and is supplied to the resonant circuit including winding L4 and capacitor C4 of the discriminator network. This resonant circuit is in turn coupled to a second resonant circuit comprising winding L3 and a capacitor C3, and a coupling capacitor C6 is connected between a center tap of winding L3 and the top terminal of winding L4. The outside terminals of the resonant circuit L3C3 are connected to rectifiers Di and D2, respectively, and the center terminal of winding L3 is connected to the junction terminal of resistors R4 and R5, the outside terminals of which resistors are connected to the output sides of the rectifiers DI and D2. The rectifiers Di and D2 may be of any suitable type and are shown conventionally. Preferably these rectifiers are of the copper oxide type but other types can be used. Consequently there is created across resistors R4 and R5 of the output of the discriminator network a direct voltage that corresponds to the frequency deviation from the center frequency of the voltage applied to tube V3 from the oscillator. This direct voltage will have a polarity according as the deviation of the frequency is above or below the center frequency of the oscillator.

The voltage applied to grid 36 of tube V3 will cause grid current to flow during the half cycle the grid is positive with respect to the cathode 32. This rectifying action of the grid circuit of tube V3 creates a voltage drop across resistors R9 and RI 8 connected to the input of the tube. The carrier frequency is by-passed from resistor RIB by capacitor C9 and a direct voltage appears across this resistor, the top terminal of resistor RI 0, that is the junction terminal of the two resistors, being negative with respect to ground.

The top or outside terminal of resistor R4 in the output of the discriminator is connected to grid 30 of the modulator tube VI by wire 42 and resistor RI, a capacitor CIZ being connected between wire 42 and ground. The junction terminal of resistors R9 and RI!) is connected to the outside terminal of resistor R5 of the discriminator network, and thus it is coupled to the grid of the modulator tube VI through the resistors R5, R4 and. RI. It is to be seen therefore that the modulator tube VI is provided with a grid bias voltage which is the resultant of the voltage derived from resistor Eli! and the voltage created through the discriminator network.

As long as the oscillator delivers the center frequency, the voltage created across the discriminator is zero and the bias voltage applied to the modulator tube VI is only that derived from resistor RI!) and this voltage remains substantially constant due to the substantially constant amplitude of the oscillations. If the oscillations deviate to rise above the center frequency, the output voltage of the discriminator combines with the voltage derived from the resistor RN! to vary the bias of the modulator tube VI and vary its reactarrce as required to bring the oscillations back to the center frequency. Similarly, if the oscillations fall below the center frequency,

the: output of the voltage of the discriminator is ofthe opposite polarity and combines with the voltage derived from resistor Hill to vary the reactance of tube VI to bring the oscillations back to the center frequency.

Also, the amplitude of the oscillations and the resulting voltage developed across resistor Hill are dependent both upon the voltage of battery it) and the voltage developed by the tuned alternator driven from battery l0. Such variation. of the voltage developed at resistor R) results in a corresponding change in the voltage applied to the modulator tube Vi tending to correct for changes in the center frequency of the oscillations due to the changes in the voltage of battery l9. That is, the automatic frequency control Voltage is dependent upon the voltage of the power supply and in this way a large degree of frequency control 'for frequency changes due to voltage changes of the power supply are obtained.

A microphone M is provided with a circuit that includes resistor R3 and low voltage battery l0. Thus a voltage varied at the audio frequency developed by speaking into the microphone is created across the resistor R3. Resistor R3 is coupled to a resistor R2 through a capacitor C8, resistor R2 and capacitor C8 forming a preempha-sis circuit. This pre-emphasis circuit is coupled to the combination of resistors R4 and R5 of the discriminator through a capacitor Cl and the audio frequencies of the microphone circuit current are applied to the control grid 30 of the modulator tube VI with the result that the center or carrier frequency of the oscillator is frequency modulated by the voice frequencies.

At the same time the elements C1--R2 and C8-"R3 form a filter network by which the audio voltages developed across the resistors R5 and R4 of the discriminator due to the frequency modulation of the carrier by the voice frequency are suppressed.

That is, the voltage developed across resistors R4 and R5 due to the slow shift of the center frequency of the oscillations because of circuit conditions is not filtered and the voltage is applied to the modulator tube but the relatively high frequency audio voltages developed across resistors R4 and R5 are by-passed by the filter network.

Apparatus such as here disclosed has the advantages that a high modulation sensitivity is effected, frequency stability of high order is obtained and pro-emphasis of the audio frequency is provided with the pre-emphasis circuit serving as a filter to suppress the audio voltages resulting from the audio frequency modulation of the carrier. Also, the apparatus is relatively simple and light to be particularly adaptable for a portable unit.

Although we have herein shown and described but one form of transmitting apparatus for a communication system using frequency modulation embodying our invention, it is to be understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In transmitting apparatus for a communication system using frequency modulation, the combination comp-rising, an electron tube oscillator to supply a carrier current of a given center frequency, a modulator electron tube connected to said oscillator to frequency modulate said carrier current according to the frequency of a com= in'unication current applied to a control grid ofthe modulator tube, a buffer electron tube, means including a capacitor to couple said oscillator to a control grid of said buffer tube to apply thereto a voltage having a frequency of said carrier current, a discriminator network connected to the output of said buffer tube to provide a direct voltage proportional to deviations of the fre--' quency of said voltage from said center frequency, a resistance load circuit including two resistors in series connected to said control grid of said buffer tube to create a direct voltage proportional to the amplitude of said carrier current, circuit means to connect the output of said discriminator and said load circuit to said control grid of said modulator tube to stabilize said center frequency of said oscillator due to the bias voltage thus applied to the modulator tube, and a filter network connected to said discriminator to by-pass the voltage created by the discriminator clue tosaid' communication current.

2. In transmitting apparatus for a communication system using frequency modulation, the combination comprising, an electron tube oscillator to supply a carrier current of a given center frequency, a modulator electron tube connected to said oscillator to frequency modulate said carrier current according to the frequency of a communication current applied to a control grid of the modulator tube, a buffer electron tube, means including a capacitor to couple said oscillator to a control grid of said buffer tube to apply thereto a voltage having a frequency of said carrier current, a discriminator network connected to the output of said buffer tube to provide a direct voltage proportional to deviations of the frequency of said voltage from said center frequency, a load circuit including a first and a second resistor in series connected between said control grid of said buffer tube and ground, a capacitor connected across said second resistor to by-pass the carrier and create a direct voltage across said second resistor due to the carrier frequency voltage applied to said bufier tube, a control circuit including said second resistor and the output of said discriminator connected to said control grid of said modulator tube to stabilize the center frequency of said oscillator by the resultant of the voltage derived from said discriminator and the voltage derived from said load circuit, and filter means connected to said control circuit to by-pass from said modulator tube voltages created by said discriminator due to the modulation of said carrier current by said communication current.

3. In transmitting apparatus for a communication system using frequency modulation, the combination comprising, an electron tube oscillator to supply a carrier current of a given center frequency, a modulator electron tube connected to said oscillator to frequency modulate said carrier current according to the frequency of a communication current applied to a control grid of the modulator tube, a buffer electron tube, means including a capacitor to couple said oscillator to a control grid of said buffer tube to apply thereto a voltage having a frequency of said carrier current, a discriminator network connected to the output of said buffer tube to provide a direct voltage proportional to deviations of the frequency of said voltage from said center frequency, a load circuit including resistance connected between said control grid of said buffer tube and ground and having a by-pass capacitor connected across at least a portion of said re-v sistance to create a direct voltage due to the grid current caused to flow in said buffer tube by the voltage derived from said oscillator, said portion of said resistance and the output of said discriminator connected in series to said control grid of said modulator tube to stabilize the centerfrequency of said oscillator due to the voltages derived -.from said discriminator and said load circuit, and filter means connected to the output of said discriminator to divert from the modulator tube the voltages created by the discriminator due to the modulation of said carrier by said communication current.

4. In transmitting apparatus for a communication system using frequency modulation, the combination comprising, an electron tube oscillator to supply a carrier current of a given center frequency, a modulator electron tube connected to said oscillator to frequency modulate said carrier current according to voltage variations applied to a control grid of said modulator tube, a buffer electron tube, a connection including a capacitor to couple said oscillator to a control grid of said buffer tube to apply thereto a voltage having the frequency of said carrier current, a discriminator network having an output including a first and a second resistor in series, said network connected to said bufier tube to create across said first and second resistors a direct voltage corresponding to deviations of the voltage applied to said buffer tube from said center frequency, a load circuit including resistance and a by-pass capacitor connected between said control grid and a cathode of said buffer tube to create a direct voltage proportional to the amplitude of the voltage applied to said control grid of said buffer tube, means to connect at least a portion of the resistance of said load circuit and said first and second resistors to said control grid of said modulator tube to stabilize the center frequency of said oscillator, a microphone circuit including a microphone and a current source, a pre-emphasis circuit including a resistor and a capacitor coupled to said microphone circuit, means including a capacitor to couple said pre-emphasis circuit to said control grid of said modulator tube to frequency modulate said carrier current by voice frequencies of said microphone, and said pre-emphasis circuit and said last mentioned capacitor forming a filter circuit network which removes the audio voltages created across said first and second resistors.

5. In transmitting apparatus for a communication system using frequency modulation, the combination comprising, a power source of a given direct voltage, an electron tube oscillator receiving energy from said power source to generate a carrier current of a given center frequency, a modulator electron tube connected to said oscillator to frequency modulate said carrier current according to voltages applied to a control grid of said modulator tube, another electron tube having a control grid capacity coupled to said oscillator to receive a voltage having the frequency of said carrier current, a discriminator network connected to said another tube to create a direct voltage corresponding to deviations of the voltage received from said oscillator from said center frequency, a resistance load circuit connected across said control grid and cathode of said another tube to create a direct voltage proportional to the amplitude of the voltage received from said oscillator, and circuit means to couple said load circuit and said discriminator to said control grid of said modulator tube to minimize deviations of the frequency of the carrier current from said center frequency due to changes in the voltage of said power source from said given direct voltage.

ADRIAN R. DOUCETTE. PAUL E. LORENTZEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 

